Prior to the present invention, various one and two-package moisture curable room temperature vulcanizable (RTV) compositions were available based on the use of a silanol-terminated polydiorganosiloxane having the formula, ##STR1## where R is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical, which is preferably methyl, or a mixture of a major amount of methyl and a minor amount of phenyl, cyanoethyl, trifluoropropyl, vinyl, and mixtures thereof and n is an integer having a value of from about 50 to about 2500, with a cross-linking silane having hydrolyzable radicals attached to silicon.
Ceyzeriat, U.S. Pat. No. 3,133,891 and Bruner, U.S. Pat. No. 3,035,016, are based on the use of methyltriacetoxysilane with a silanol-terminated polydimethylsiloxane under substantially anhydrous conditions. Although the one-package compositions of Bruner or Ceyzeriat, upon exposure to atmospheric moisture, provide satisfactory one-package room temperature vulcanizable organopolysiloxane compositions exhibiting satisfactory tack-free time, for example, 30 minutes or less after an extended shelf period, the acetic acid by-product is corrosive and has a disagreeable odor.
Other variations of one-package acyloxy acid generating RTV's are shown by Kulpa, U.S. Pat. No. 3,296,161, Goossens, U.S. Pat. No. 3,296,195 and Beers, U.S. Pat. No. 3,438,930, assigned to the same assignee as the present invention. Additional one-package acyloxy acid generating RTV compositions are shown by Schulz et al, U.S. Pat. No. 3,647,917 and Nitzsche et al U.S. Pat. No. 3,886,118.
An improved, low odor, substantially non-corrosive one-package RTV composition is shown by Beers, U.S. Pat. No. 4,257,932, assigned to the same assignee as the present invention. Beers achieves a reduction in odor and corrosive properties by utilizing as a cross-linking silane, a less volatile material such as methyltris-(2-ethylhexanoxy)silane.
A non-corrosive two package moisture curable organopolysiloxane composition free of carboxylic acid generating groups is shown by Nitzsche et al, U.S. Pat. No. 3,127,363 which is based on the use of a polyalkoxysilane, or polysilicate cross linking agent, in place of methyltriacetoxysilane. The ingredients of the two package noncorrosive composition of Nitzsche et al, are mixed under atmospheric conditions and the resulting composition must be used soon after the ingredients are mixed because the resulting blend has a short shelf life. Although the mixture of Nitzsche et al, which is typically polyalkoxysilane, silanol-terminated polydiorganosiloxane and tin soap catalyst, provides upon mixing, a fast curing non-corrosive room temperature vulcanizable composition, the Nitzsche et al mixture does not have the extended shelf life advantage of the one package system which is required for various commercial uses and therefore is excluded from a variety of applications.
Nitzsche et al, U.S. Pat. No. 3,065,194, teaches that a mixture of an endblocked dimethylsiloxane polymer, such as hydroxy and alkoxy endblocked, inert filler, ethylorthosilicate and dibutyltindilaurate can be vulcanized upon contact with water, after a 14 day shelf period at room temperature. However, the various ingredients of the mixture have to be vigorously dried by heating for 1 hour at 200.degree. C., and the RTV, after a relatively short shelf period, has to be drenched with water.
Improved results toward combining the advantages of a non-corrosive acid-free polyalkoxysilane cross-linking agent with a silanol-terminated polydiorganosiloxane as a one-package system are shown by Weyenberg, U.S. Pat. No. 3,334,067, Cooper et al, U.S. Pat. No. 3,542,901 and by Smith et al U.S. Pat. Nos. 3,689,454, and 3,779,986, the last two being assigned to the same assignee as the present invention, utilizing a titanium chelate catalyst in place of a tin catalyst. However, after room temperature vulcanizable one-package systems based on a titanium chelate catalyst were allowed to age for a period of 5 hours or more, it was found that the tack-free time of the aged RTV was considerably longer than the tack-free time of the same mixture after it was initially mixed and immediately exposed to atmospheric moisture.
As shown by Brown et al U.S. Pat. No. 3,122,522, a platinum catalyst is used to prepare an alkoxy terminated silalkylenepolysiloxane polymer. However, this method of synthesizing the base polymer requires an expensive hydrosilylation procedure. Additional efforts to achieve a desirable non-corrosive, substantially odor-free stable one-package RTV based on the use of polyalkoxy organo polysiloxane in a more economic manner are shown by Brown et al, U.S. Pat. No. 3,161,614 or U.S. Pat. No. Re. 29,760. Brown et al employed a polyalkoxy end blocked polysiloxane which was based on the use of a mineral acid generating polyalkoxyhalosilane, and a curing catalyst. However, these compositions were found to be unusable because they failed to cure in contact with a tin catalyst, even in the presence of moisture.
As utilized hereinafter, the term "stable" as applied to the one package alkoxy-terminated organopolysiloxane RTV's of the present invention means a moisture curable mixture capable of remaining substantially unchanged while excluded from atmospheric moisture and which cures to a tack-free elastomer after an extended shelf period. In addition, a stable RTV also means that the tack-free time exhibited by freshly mixed RTV ingredients under atmospheric conditions will be substantially the same as that exhibited by the same mixture of ingredients exposed to atmospheric moisture after having been held in a moisture resistant and moisture-free container for an extended shelf period at ambient conditions, or an equivalent period based on accelerated aging at an elevated temperature.
Valuable, substantially acid-free cured products have been obtained with the use of particular organosilane scavengers, as taught in the afore-described application of White et al, Ser. No. 277,524 now U.S. Pat. No. 4,395,526, for One Package, Stable, Moisture Curable, Polyalkoxy-Terminated Organopolysiloxane Compositions and Method for Making. Likewise, as shown in White et al, copending application Ser. No. 481,524, filed concurrently herewith, silicon-free organic scavengers have been found to provide substantially acid-free cured products from RTV compositions which in many instances are more economical than those shown in Ser. No. 277,524 now U.S. Pat. No. 4,395,526.
It has been found that although these organosilanes or silicon-free organic materials can serve as scavengers for materials having chemically combined hydroxy radicals in the RTV compositions, an amine accelerator is generally required in combination with such scavengers to achieve a stabilized RTV composition which has a satisfactory cure time.
The present invention is based on the discovery that stable, substantially acid-free, one package moisture curable RTV compositions can be made by utilizing as the scavenger for chemically combined hydroxy radicals, an N-silyl-substituted imidazole of the formula, ##STR2## which has been found to be effective as both a stabilizer and cure accelerator of substantially acid-free, fast curing, polyalkoxy-terminated organopolysiloxane compositions, where R.sup.1 is selected from a C.sub.(1-13) monovalent hydrocarbon radicals and substituted C.sub.(1-13) monovalent hydrocarbon radicals, R.sup.2, R.sup.3 and R.sub.4 are selected from hydrogen, C.sub.(1-8) alkyl, C.sub.(1-8) alkenyl and C.sub.(6-13) aryl and a is an integer equal to 1-4 inclusive.
Accordingly, the above-described N-silyl-substituted imidazoles of formula (2) can be advantageously used in combination with alkoxy-terminated polydiorganosiloxane of the formula ##STR3## where R and n are as previously defined, R.sup.5 is a C.sub.(1-8) aliphatic organic radical selected from alkyl radicals, alkylether radicals, alkylester radicals, alkylketone radicals and alkylcyano or a C.sub.(7-13) aralkyl radical, R.sup.6 is a C.sub.(1-13) monovalent organic radical selected from R radicals as previously defined, and b is a whole number equal to 0 to 2 inclusive. It has been found that there can be utilized from about 0.1 to about 10 parts of the N-silyl substituted imidazole of the present invention per 100 parts of the alkoxy-terminated polydiorganosiloxane of formula (3) or the silanol terminated polydiorganosiloxane of formula (1). Preferably, 1 or 2 parts of the N-silyl substituted imidazole per 100 parts of organopolysiloxane polymer can be used.
Although the N-silyl imidazoles of formula (2) can function as both a stabilizer and curing accelerator for the room temperature vulcanizable alkoxy-terminated organopolysiloxane compositions of the present invention, in particular instances curing accelerators such as certain organic amines can be used in combination with the N-silyl imidazoles to further enhance the cure rate of the resulting RTV composition. Furthermore, there can be utilized in the room temperature vulcanizable compositions of the present invention cross-linking polyalkoxysilane having the formula, ##STR4## where R.sup.5 and R.sup.6 are as previously defined and c is a whole number equal to 0 or 1.